1. Field of the Invention
The present invention relates to a voltage conversion device, and more particularly, to a voltage conversion device having a non-linear gain.
2. Description of the Prior Art
A liquid crystal display (LCD) monitor has characteristics of light shape, low power consumption, zero radiation, etc. and has been widely used in many information technology (IT) products, such as computer systems, mobile phones, and personal digital assistants (PDAs). The operating principle of the LCD is based on the fact that liquid crystals in different twist status can result in different polarization and refraction effects on light. Thus, the liquid crystals can control penetration amount of light by arranging in different twist status, so as to produce various brightness of light output and diverse gray levels of red, green and blue lights.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a thin-film-transistor (TFT) LCD monitor 10 according to the prior art. The LCD monitor 10 includes an LCD panel 100, a control circuit 102, a data-line-signal output circuit 104, a scan-line-signal output circuit 106, and a voltage generator 108. The LCD panel 100 is formed with two substrates, and LCD layers are stuffed between the substrates. One substrate includes a plurality of data lines 110, a plurality of scan lines (or gate lines) 112 perpendicular to the data lines 110, and a plurality of TFTs 114. The other substrate includes a common electrode for providing a common voltage Vcom generated by the voltage generator 108. For the sake of brevity, FIG. 1 reveals only four TFTs 114, but in a real case, there is one TFT 114 set at each intersection of a data line 110 and a scan line 112 on the LCD panel 100. In other words, the plurality of TFTs 114, each corresponding to a pixel, form a matrix on the LCD panel 100, and the data lines 110 and the scan lines 112 are corresponding to columns and rows of the matrix. In addition, a circuit effect resulted from the two substrates of the LCD panel 100 can be regarded as equivalent capacitors 116.
A driving process of the prior art TFT LCD monitor 10 is described in details as follows. When the control circuit 102 receives a horizontal synchronization signal 118 and a vertical synchronization signal 120, the control circuit 102 generates corresponding control signals for the data-line-signal output circuit 104 and the scan-line-signal output circuit 106. The data-line-signal output circuit 104 and the scan-line-signal output circuit 106 generate input signals for the data lines 110 and the scan lines 112 according to the control signals, in order to control the TFTs 114 and voltage differences of the equivalent capacitors 116. The voltage differences change twist of liquid crystals and corresponding penetration amount of light so as to display the display data 122 on a panel. For example, the scan-line-signal output circuit 106 outputs a pulse wave for turning on a TFT 114, and signals of a corresponding data line 110 outputted from the data-line-signal output circuit 104 can pass through the TFT 114 to a corresponding equivalent capacitor 116, so as to control a gray level of corresponding pixel. Besides, signal level of the signals on the data line 110 outputted from the data-line-signal output circuit 104 drives a gray level of corresponding pixel.
In the prior art, in order to reduce EMI and save power under high-frequency operations, a voltage swing outputted from the display data 122 is basically very small, i.e., 0.1V˜1V. Therefore, signals inputted to the data-line-signal output circuit 104 pass through a voltage conversion circuit, which adjusts voltage swings of the inputted signals to a predetermined range, so as to control chrominance and luminance of corresponding pixels. For example, please refer to FIG. 2. FIG. 2 is a schematic diagram of a voltage conversion circuit 20 according to the prior art. The voltage conversion circuit 20 includes an operational amplifier 200 and resistors 202, 204. The operational amplifier 200 has an input end 206 coupled to an output end of the display data 122, and an output end 208 coupled to an input end of the data-line-signal output circuit 104. As those skilled in the art recognized, if the operational amplifier 200 is an ideal operational amplifier having an infinite gain while the resistance of the resistors 202, 204 are R1 and R2, the gain of the voltage conversion circuit 20 is (1+R2/R1). In other words, the input and output voltages of the voltage conversion circuit 20 relates to a linear function.
Therefore, data signals outputted from the display data 122 are amplified via the voltage conversion circuit 20 and then are outputted to the data-line-signal output circuit 104. Under this circumstance, data-line signals outputted from the data-line-signal output circuit 104 are amplified to an operating range of the TFTs 114 via the prior art voltage conversion circuit 20, so that gray level and brightness revealed on the TFT LCD monitor 10 approximately relates to a linear line. Therefore, in the prior art, this effects color gradient of images, resulting in unnatural images.